A fiber optic sensor unit can be used in a vortex shedding flowmeter. In such a flowmeter, a sensor beam is exposed to a flow of fluid that has fluid vortices generated therein by an obstruction in the flow path. The frequency of the fluid vortex generation (called shedding) is a measure of the flow rate for the fluid. Each time a vortex is shed, the sensor beam is moved. This movement is transferred to one of the many known assemblies used for changing the characteristics of light transmitted and received through a fiber optic cable. Such assemblies include microbend jaws which bend the optical fiber or cable modulating the light intensity therein and divided optical fiber assemblies having one of the fiber ends moving in response to the sensor beam, eliminating a portion of the light travelling through the fiber.
It is well known in the art to provide a control system in order to control a vortex shedding flowmeter. A conversion unit must be provided as part of the control system to convert the light signals transmitted through the fiber optic cables into an electrical signal that is compatible with the control system. A communication path must also be provided as part of the control system to transmit the converted electrical signals to a central control unit.
One known and industrially accepted system for conveying signals from a transducer, such as a flowmeter, to a central control unit is a two wire, 4-20 ma analog transmission system. It is known in the art to use a two wire, 4-20 ma transmission system for fiber optic vortex shedding flowmeters, as disclosed in U.S. Pat. No. 4,655,353 (Thompson).
In the analog transmission system the modulated light intensity in the optical cable is converted to an analog current or voltage proportional to the flow indicated by the sensor beam. These analog transmission systems are limited in vortex shedding flowmeter applications. The vortices shed are a non-linear quantity and non-linear signals limit the range of accurate flow signals that can be transmitted on a 4-20 ma path. There are also hysteresis and thermal drift effects in the analog circuitry that limit the repeatability, and therefore the accuracy, of the vortex shedding flowmeter signal that is transmitted through a 4-20 ma path. An analog transmission path cannot provide control signal communication between the flowmeter and a control unit; it can only provide one way communication by transmitting the flow as sensed by the flowmeter to the central control unit. No control signals can be sent from the central control unit to the flowmeter.
The conversion units used in fiber optic vortex shedding flowmeter control systems have also limited the range of vortex shedding flowmeters. Preamp circuits used in present conversion units, such as disclosed in U.S. Pat. No. 4,628,197 (Thompson), are not able to sense low-end signals generated by a vortex shedding flowmeter. Thus, a digital control system for a fiber optic vortex shedding flowmeter is needed which makes adjustments for the non-linearity of the vortex shedding phenomena and corrects or eliminates thermal effects and hysteresis. It has become desirable to provide two-way digital communication between the fiber optic vortex shedding flowmeter and a control unit while also improving the low end sensitivity of the conversion circuitry used in fiber optic vortex shedding flowmeters.